The present invention relates to fluid impellers used with mixed flow fans such as fans for moving air or other gases.
Various motor-driven fan configurations have been proposed to meet respective different requirements for performance, noise generation, and cost, etc. A better understanding of these motor-driven fan configurations may be obtained from the following description which references FIGS. 1 through 7.
FIGS. 1 and 2 show schematic end and cut-away side views respectively of an axial flow fan blade assembly. The impeller includes a number of aerofoil blades 10 fixedly mounted to a central hub 20, which is generally enclosed within a stationary cowling 30. The impeller is driven by a motor 40, and air is driven by the impeller in a direction 50 which is substantially along the axis of rotation of the fan blade assembly.
Axial flow fans provide large volume flow rates of air, but operate at relatively low pressures. As the pressure increases, the fan is liable to stall.
FIGS. 3 and 4 of the accompanying drawings show schematic end and cut-away side views of a prior art centrifugal fan. This type of fan comprises blades 60 fixedly mounted to a rotating hub 70 driven by a motor 80. The centrifugal fan has a casing 90 which allows air to enter generally along the axis of rotation of the blade assembly but to exit perpendicular to the entry direction.
In the centrifugal fan, air is forced to rotate by movement of the blades 60 and is flung outwards towards the exit port 100 by the centrifugal effect. Centrifugal fans are recognized for their low volume flow rates of air but high pressure performance, generally without the stalling problems exhibited by axial flow fans. However, centrifugal fans are generally not suitable for use with large volume flow rates.
The so-called mix flow fan was developed as a compromise between the axial and centrifugal fan assemblies. It is designed to operate at generally higher pressures than an axial flow fan, but to provide a generally greater volume flow rate than a centrifugal fan. FIGS. 5 and 6 are schematic and cut-away side views respectively of a prior art mixed flow fan.
The mixed flow fan comprises a number of blades 110 attached to a central frusto-conical hub 120 and to a generally frusto-conical shroud 130. The blades 110, hub 120, and frusto-conical shroud 130 form a complete rotating assembly, driven by a motor 140.
In operation, the fan behaves as a combination of the axial and centrifugal flow devices, so that air entering the shroud 130 is drawn into the impeller, with a velocity component along the axis of rotation, but the air is also driven outwardly in a similar manner to the centrifugal fan, with a velocity component perpendicular to the axis of rotation. These two velocity components combine to give an output direction 150 as illustrated in FIG. 6.
FIG. 7 is a typical pressure-volume flow rate performance graph comparing the performance of the prior art fans shown in FIGS. 1 through 6.
In FIG. 7, the first curve 160 illustrates the high-pressure, low volume performance of a centrifugal flow fan. The second curve 170 represents the high-volume, low-pressure operation of an axial flow fan. (The stalling characteristic of the axial flow fan is not shown on FIG. 7.) The third curve 180 shows the performance of a mixed flow fan which provides a generally higher volume but lower pressure performance in comparison to the centrifugal fan, and a higher pressure but lower volume performance in comparison to the axial flow fan.
Each of the performance curves shown schematically in FIG. 7 relates to a particular prior art fan configuration (fan diameter, number of blades and angle of blades) and rotation speed of the blade assembly. Once these fan characteristics have been set, the fan performance is generally fixed, so that, for example, if the operating pressure for the fan is specified, the resulting volume flow-rate which will be obtained is defined by the performance curve.
However, it is desirable in manufacturing and installing fans to be able to vary the performance of the fans. This allows a manufacturer to market a range of fans having different performance curves, but which share some or all of their components in common.
In the case of an axial flow fan, it is relatively easy to vary the fan""s performance while still using the same mechanical components. For example, the blade angle of incidence can be varied to give dramatic changes in the performance characteristics. In one example, a change in the blade angle of incidence from, say, 10xc2x0 to 40xc2x0 could result in 2:1 change in volume flow rate (and a correspondingly large change in driving power consumption).
However, in the centrifugal and mixed flow fans described above, there is little room for changing the fan""s performance. The number of blades can be varied, but this tends to give dramatic, rather than gradual, changes in performance. The motor speed can be varied, but this requires either a belt drive system, which adds to the mechanical complexity of the fan, or the use of different motors, such as two-pole, four-pole, six-pole motors, etc. However, since the rotation speed of a two-pole motor is twice that of a four-pole motor, this again leads to dramatic, rather than gradual, variations in the fan""s performance.
In summary, none of the previously proposed fans described above provide relatively high pressure operation and still allow the fan performance to be easily varied.
This invention provides a mixed flow fan which provides relatively high pressure operation while still allowing the fan performance to be easily varied. Specifically, the fluid impeller of the fan of the present invention includes one or more rotatable fluid impelling blades and a hub adjacent to an edge of the blades. At least one blade facing surface on the hub is formed substantially as a segment of a sphere of radius r1 about a center of curvature. Surrounding the blades is a shroud which is adjacent to an outer edge of each blade. At least one blade facing surface of the shroud is formed substantially as a segment of a sphere of radius r2 about the center the curvature. Each blade has a hub abutting edge curve which substantially fits against a sphere of radius r1 and a shroud abutting edge curved substantially to fit against a sphere of radius r2 so that each blade is attachable to the hub and to the shroud at various angles about an axis passing through the center of curvature while the hub abutting and shroud abutting edges of the blade remain substantially abutting the hub and the shroud respectively.